There are various types of nonvolatile memories formed on semiconductor substrates. Among those, a flash memory, which stores information by accumulating electrons in a floating gate, is generally and widely used due to the merits that higher integration can be achieved.
Such flash memory includes an ONO film and a control gate which are formed on a floating gate. Of these films, unnecessary ONO film at portions between cells is desired to be removed by etching. In reality, however, some portions of the ONO film between the cells are not etched due to a step of an underlying layer, so that the etching residues of ONO film remain.
For example, in Japanese Laid-open Patent Publication No. 05-267683, in order to solve a problem that a residue of an ONO film causes an electric short-circuit between adjacent floating gates and thereby lowers a yield, the ONO film is formed after insulating side walls are formed on side surfaces of the floating gates. In this way, the etching residue of the ONO film is unlikely to be left.
However, when the insulating side walls are newly formed in this manner, the number of manufacturing processes is increased, which in turn increases the manufacturing cost of a semiconductor device.